The preliminary version of a nonlinear model describing the major pathways of adenosine transport and metabolism in the heart will be extended and improved to include new features that are needed to increase the general utility and dissemination of the model. The model structure will be rewritten to bring it into conformity with the usual Resource standards for exportable models, including model structure, documentation and testing routines. The implementation of multiple flow pathways will be improved, vascular operators will be added to describe non-exchanging vessels, the rate laws for two of the enzymes in the model will be changed to include competitive inhibition by selective enzyme blocking agents, and a new species will be added to the model to calculate inosine concentrations. Finally, the model will be reconfigured to run under the new simulation interface, XSIM, to increase the efficiency of the model for the user and make for more general dissemination. This model will be u sed in the final Specific Aim of section D.II.B.2 to incorporate the myocardial high energy phosphate model with a refined description of the transport and metabolism of adenosine in capillary endothelial cells and cardiomyocytes.